Tinidazole milk excretion and pharmacokinetics in lactating women

Five women undergoing acute Caesarean section were given an i.v. dose of 1600 mg tinidazole preoperatively as prophylaxis against anaerobic infection. Blood and breast milk samples were collected at 8 and 4 h intervals, respectively, for 120 h. Tinidazole concentrations were measured by means of high performance liquid chromatography (h.p.l.c). The concentration of tinidazole in breast milk was highly related to the concentration in serum (r = 0.969). Tinidazole concentrations in serum declined monoexponentially with an average half-life of 11.4 h (range 8.7-13.1). The milk/serum concentration ratio varied between 0.62 and 1.39. Seventy-two hours after the Caesarean section the milk concentration exceeded 0.5 microgram/ml in only one woman. It may be calculated that at this time the maximum daily dose to the infant would be 0.1 mg/kg body weight (assuming 3.5 kg body weight and 400 ml milk consumed). We conclude that until tinidazole has been proven harmless to the neonate breast feeding following i.v. administration of 1600 mg should not be initiated earlier than 72 h after the dose.